Injection molding systems have been developed having flow control mechanisms that control the movement of a valve pin over the course of an injection cycle to cause the pin to move either upstream or downstream over the course of an injection cycle in order to raise or lower the rate of flow of fluid material into the cavity to correspond to a predetermined profile of fluid flow rates over the length of the injection cycle. A sensor is used to sense a condition of the fluid material or of the apparatus, the sensor sending a signal indicative of the sensed condition to a program contained in a controller that uses the signal as a variable to control movement of the valve pin in accordance with the predetermined profile.